Treatment of regenerated cellulose



Patented Dec. 30, 1941 TREATMENT OF'REGENERATED CELLULOSE Ralph J. Quaid, Sayreville, N. J assignor to E. I. du Pont de Nemours & Company, Wilmington, DeL, a corporation of Delaware No Drawing.

Application May 6, 1939,

Serial No. 272,143

6 Claims.

This invention relates to a process for reducing the solution viscosity of regenerated cellulose and more particularly to a process for reducing the solution viscosity of regenerated cellulose adapted for use in the manufacture of cellulose nitrate of low viscosity characteristic.

Commercial utilization of cellulose nitrate coating compositions received greatest impetus with the discovery of a process for reducing the viscosity characteristic of cellulose nitrate, thus permitting the use of higher concentrations of this ingredient with resultant thicker films with each application of the coating. Numerous processes have been proposed for reducing the viscosity characteristic of cellulose nitrate and some of these processes are successfully employed on a commercial scale at the present time. The desired reduction may be accomplished by suitable digestion of the nitrated cellulose in an autoclave at elevated temperatures and pressures, but this method is. hazardous and requires expensive equipment for its operation. The cellulose nitrate may be treated with ammonia according to the process described in U. S. Patent 2,039,381, but it is sometimes difficult to completely remove the residual ammonia bodies which may cause corrosion and perhaps gelation in some solutions. Cellulose nitrate of low viscosity characteristic has also been made by nitrating cellulose at high temperatures but such treatment causes low yields and high acid losses.

Alternatively it has been possible to reduce the solution viscosity of fibrous cellulose prior to nitration from which a cellulose nitrate of low viscosity characteristic may be manufactured. A treatment with hydrochloric acid or equivalent hydrogen halide provides a fibrous cellulose of low solution viscosity but this method tends to deleteriously degrade the cellulose and, furthermore, introduces serious equipment corrosion problems. To accomplish the desired objective, it has also been proposed to treat cellulose fibres with certain oxidizing agents such as permangahates, hypochlorites, and peroxides. Economic considerations preclude commercial operation of a process involving treatment with these agents and, in addition, results are erratic and very carefully controlled conditions are required.

U. S. Patent 1,997,766 describes the manufacture of low viscosity cellulose nitrate by the direct nitration of regenerated cellulose to afiord a product which has received widespread commercial acceptance. The cuprammonium viscosity of the regenerated cellulose prior to nitration is about 2 compared to water as 1 and the viscosity characteristic of the nitrated product prepared in accordance with the patented process is about ,4) second Hercules (about centipoises). As far as known, no method has heretofore been devised whereby a viscosity characteristic below that obtained according to the patent for the particular raw material therein described may be secured.

Synthetic resinous coating compositions, particularly those based on alkyd type resins, have become an important competitive factor in the finishing industry, thereby indicating the need for even greater solids concentrations in cellulose nitrate coating compositions than have heretofore been employed if serious curtailment in their use is to be avoided. Previous practice provided for the use of cellulose nitrate of second Hercules viscosity characteristic (about 120 centipoises) and most of the above described processes were employed in producing that type of material but in order to permit the inclusion of greater proportions of cellulose nitrate, a fibrous type of cellulose nitrate of A second viscosity characteristic (about 50 centipoises) has been employed.

The present invention, therefore, is directed to a process for reducing the solution viscosity of regenerated cellulose (a non-fibrous, amorphous type of cellulose) to provide a material which may be employed for the manufacture of cellulose nitrate of second or lower viscosity characteristic by direct nitration without resorting to abnormally high nitrating temperatures which tend to give low yields and high acid losses.

The invention has as a primary object the provision of a process for reducing the solution viscosity of regenerated cellulose. Another object is the provision of a process for reducing the solution viscosity of regenerated cellulose which is simple, economical and non-hazardous, and which affords particularly uniform results without elaborate precautionary control. Another object is the provision of a process for reducing the solution viscosity of regenerated cellulose without expensive equipment and in the absence of serious corrosion problems. A further object of the invention is the provision of a process for reducing the solution viscosity of regenerated cellulose in which only moderate temperature and relatively short treating cycles are required, and by which there is no degradation or destruction of the regenerated cellulose. Another object is the provision of a new product consisting of regenerated cellulose characterized by a cuprammonium viscosity of less than 2 compared to water as 1. A further and ancillary object is the provision of a process for reducing the solution viscosity of regenerated cellulose whereby cellulose nitrate of viscosity characteristic of A second and lower may be obtained by direct nitration. Other objects will appear as the description of the invention proceeds.

These objects are accomplished by treating regenerated cellulose, preferably comminuted regenerated cellulose sheeting, with dilute nitric acid or with dilute spent acids recovered from the nitration of cellulose and consisting of a mixture of nitric and sulfuric acids.

In the practice of the invention, the comminuted regenerated cellulose, which may be scrap material previously comminuted and treated to remove coating and glycerine in accordance with the process described in U. S. Patent 1,997,766, is steeped in a dilute aqueous, solution of nitric or spent acids recovered from the nitration of cellulose and containing nitric acid as an essential ingredient, at moderately elevated temperatures for a suiiicient time to aflord the desired reduction in the solution viscosity of the regenerated cellulose.

The following examples illustrate the preferred procedure but it will be understood that the invention is not limited thereby, except as indicated in the appended claims, since variations will occur to those skilled in the art without departing from the spirit of the invention.

Example 1 The material employed in the process of this example consisted of scrap regenerated cellulose sheeting resulting from the trimming of rough edges, cutting of sheets into certain sizes and other practices followed in the manufacture of commercial regenerated cellulose sheeting. This material contains contaminants such as glycerine, commonly used as a softener for the sheeting, and sometimes coloring matter and lacquer coatings to afford moistureproofing, etc. These contaminants are, therefore, removed and the material comminuted prior to the treatment of the present invention by any convenient means such as, for example, as described in detail in I U. S. Patent 1,997,766. The method for comminuting the regenerated cellulose sheeting provided in this patent is preferred in the present invention since it also greatly facilitates the efficacy of the viscosity reducing agent by preventing the laminating of the discrete particles through the curling or crinkling efiect produced thereby. The dilute sodium hydroxide solution employed in the first step of the process of the patent does not act to purify or chemically or physically alter the comminuted regenerated sheeting since the material is already in a highly purified state through treatments given in the manufacture of the regenerated cellulose, and in this instance, the alkali merely serves to facilitate removal of the physically added contaminants. As pointed out in the patent, the use of the sodium hydroxide is optional and not essential to the successful working of the process.

Three hundred grams of highly purified comminuted regenerated cellulose sheeting now free of added extraneous material with a moisture content of about 2%, were charged into a stainless steell vessel containing 4500 grams of a spent nitrating acid consisting of nitric and sulfuric acids in the proportion of 30% nitric acid and 50% sulfuric acid diluted with water to an acidity of 0.6% expressed as nitric acid previously heated to 70 C. The charge was steeped at this temperature for 4 hours after which it was washed with water until neutral and dried.

The cuprammonium viscosity of the comminuted regenerated cellulose prior to the above described treatment was 2 compared to water as 1 and the soda soluble content was 10%. The treated product was characterized by a cupammonium viscosity of 1.35 compared to water as 1 and the soda soluble analyzed 26.5%.

The actual source of the spent acids from nitrating baths which may be used in the proces of the invention is not of particular materiality as long as there are no contaminants present which would adversely affect the regenerated cellulose and nitric acid is present as an essential ingredient, preferably in an amount not less than about the amount of sulfuric acid in the mixture. As in the case of straight nitric acid, the concentration is adjusted to not more than about 2% (calculated as nitric acid).

This product was then nitrated in accordance with conventional practice by immersing 200 grams of the material with continuous agitation in 7200 grams of a nitrating acid mixture of the following composition:

Per cent HNOa 30.3 H2304. 50.3 mo 1 19.4

at a temperature of 47 C. for 45 minutes. The spent acid was removed by centrifuging and the cellulose nitrate stabilized by the conventional acid boils and washings. The viscosity characteristic of the final product was 5.0 seconds (about V second Hercules) as determined according to A. S. T. M. Specification D-301-33, Formula C. Expressed in centipoises, the viscosity is 56.6 centipoises. The nitrogen content of the product was 11.99%.

The viscosity characteristic of a cellulose nitrate prepared from untreated comminuted regenerated cellulose consisting'of the same starting material as employed in Example 1 and using the same nitrating acid composition was 12 seconds (about second Hercules) as determined according to A. S. T. M. Specification D-301-33,

Three hundred grams of comminuted regenerated cellulose sheeting treated to remove extraneous material as in Example 1 were immersed in 4500 grams of dilute nitric acid solution of 1.2% concentration at a temperature of 70 C. The charge was steeped for a period of 4 hours, washed until neutral and dried to a moisture content of about 2%. The cuprammonium viscosity of this product was 1.19 and the soda soluble 48.1%

The product was nitrated in accordance with the procedure shown for nitrating the product of Example 1, except that the nitrating temperature was raised'to 58 C. The viscosity characteristic of the cellulose nit-rate thus obtained was 0.9 second (10.2 centipoises). The nitrogen content of the product analyzed 11.95%.

Eaiample 3 Three hundred grams of comminuted regenerated cellulose sheeting treated to remove extraneous material-as in Example 1 were immersed in 4500 grams of aqueous nitric acid solution of 1.2% concentration at a temperature of 70 C. The charge was steeped at this temperature for a period of 4 hours, washed until neutral and dried to a moisture content of 2%. Thecuprammonium viscosity of this product was 1.30 and the soda soluble 26.5%.

This material was nitrated in accordance with the procedure shown for nitrating the product of Example 1. The viscosity characteristic of the cellulose nitrate thus obtained was 4.6 seconds (about A second Hercules). Expressed in centipoises, the viscosity is 52.0 centipoises. The nitrogen content of the product was 11.95%.

The method of determining the cuprammonium or solution viscosity of the regenerated cellulose was as follows: Dudley pipettes of 100 c. 0. capacity, giving drainage times of 33-35 seconds for water at 25 C. were selected and standardized using distilled water. During the tests, the pipettes were maintained at 25 C. by a surrounding water jacket. n measuring the solution viscosity of a sample of regenerated cel lulose, 3.05 grams of cupric hydroxide, 5 grams of regenerated cellulose and 225 c. c. of ammonium hydroxide (sp. gr., 0.96) at 25 C. were introduced into an 8 oz. wide mouth bottle, the bottle closed by means of a cork, and the mixture tumbled overnight. The solution was then placed in a water bath maintained at 25 C. for one hour after which it was drawn into the pipette and the time of draining of 100 c. c. measured with a stop watch. The specific solution viscosity is calculated by reference to .the draining time of water.

While it is preferable to remove contaminants such as glycerine or lacquer coating which may be present in scrap regenerated cellulose sheeting prior to the viscosity reducing treatment, it is also possible to obtain satisfactory results by applying the treatment direct to the comminuted material containing the aforementioned contaminants. In this manner the glycerine and lacquer coating may be removed simultaneously with the reduction in the solution viscosity of the regenerated cellulose.

The process permits a rather wide temperature range governed largely by the time of treatment and the concentration of the acid solution. For most purposes temperatures of between 20 C. and 100 C. are satisfactory although a range of between 60 C. and 80 C. is preferred with. the preferred time cycles, acid concentrations, and solution viscosity as indicated below.

Operating economies are apparent in the new process because of the relatively short treating cycles required to give the desired reduction in the solution viscosity, and for most cases about 4 hours sufiices unless the treatment is carried out at lower and less effective temperatures. Under some conditions a treatment of about 1 hour is satisfactory. However, longer treating cycles are not precluded provided no destructive effects are occasioned thereby.

Acid concentrations of between 0.1% and 2.0%, expressed as nitric acid, regulated by the temperatureand treating time may be used. Optimum conditions include an acid concentration of about 0.5 %1.5% as applied to the regenerated cellulosefor a period of about 4 hours at a steeping temperature of about 70 C.

In the examples, the ratio of acid treating solution to regenerated cellulose is given as 15 to 1. Although these proportions appear to represent the optimum for the particular conditions described, the process may also be operated successfully where this ratio may be varied between 10 and 30 or more parts of treating solution to 1 part of regenerated cellulose.

A four hour treating cycle affords the desired reduction in solution viscosity of the regenerated cellulose under the prescribed conditions shown in the examples, but with variations in the temperature and acid concentration to give a more stringent treatment, this time may be reduced to one hour or less and if the treating conditions are moderated the time of treatment should be increased for a period sufficient to give the desired viscosity characteristic.

Although waste or soap regenerated cellulose is preferably employed in the process of the present invention from economic considerations, the use of regenerated cellulose which is not scrap or' waste material is not precluded, since the invention is equally applicable with the regularly manufactured material.

As far as is known, the herein described process as employed for the reduction in the solution viscosity of comminuted regenerated cellulose to provide a material that will afford cellulose nitrate with a viscosity characteristic of substantially below /2 second Hercules is new and novel and offers a distinct and highly desired advance in the art. Certain mineral acids such as HCl and H2804 have been proposed for the purification of fibrous cellulose to remove gummy constituents from the fibres by hydrolyzing such materials. However, in the regenerated cellulose which is quite non-fibrous in character employed in the present invention, no such materials are present, since the regenerated cellulose is already a highly purified material. HCl and other hydrogen halides have been suggested for reducing the solution viscosity of fibrous cellulose but these acids including sulfuric acid if used in sufficient concentrations to achieve the results of the present invention cause serious degradation of cellulose and, in addition, present difficult corrosion problems. Prior proposals are not directed to methods for reducing the solution viscosity of regenerated cellulose which in its ordinary existent state is characterized by a low solution viscosity on the order of that generally secured by prior art treatments applied to fibrous cellulose. In the new process there is no degradation of the regenerated cellulose and corrosion problems are relatively insignificant.

The process of the present invention is primarily designed to provide cellulosic material of low cost and of such character that cellulose nitrates of very low viscosity characteristics (below /2 second Hercules) may be manufactured by direct nitration without requiring any subsequent treatment to alter the viscosity characteristic.

The advantages of the new process are several. A simple and direct means is offered whereby regenerated cellulose of very low solution viscosity may be manufactured, the product lulose nitrate (about 50 centipoises), including also cellulose nitrates or still lower viscosity characteristic such as indicated in Example 2 where the product has a viscosity 10.2 centipoises. The economics of the invention are immediately apparent in view of the use of waste regenerated cellulose which may be treated with dilute concentrations of spent acids recovered from previous nitration operations. Further economies are secured through the short treating cycles and the moderate temperatures employed in the new process. Also, no expensive or complicated equipment is needed for the operation of the invention. In contrast to prior proposals, the present treatment does not tend to degrade the regenerated cellulose. The process is also non-hazardous and does not require critical control in its operation. Cellulose nitrate of below 5: second Hercules viscosity may be produced by direct nitration, thus avoiding the necessity of further treatment to provide a product of this lower viscosity characteristic range. The invention also extends the usefulness of a waste material and, in addition, provides a means for manufacturing a product which permits a higher solids content in cellulose nitrate coating compositions, a property which is quite desirable in the finishing industry.

As many widely different embodiments of the invention may be made without departing from the spirit and scope thereof, it will be understood that the invention is not limited to the aaeaoea specific embodiments thereof except as defined in the appended claims.

I claim:

1. Process for reducing the solution viscosity oi regenerated cellulose which comprises treating pure comminuted regenerated'cellulose sheeting with a dilute aqueous nitric acid containing so-- lution of between 0.1 and 2.0% concentration (expressed as nitric acid) and containing nitric acid as an essential ingredient for about 4 hours between about 20 C. and 100 C.

2. Process of claim 1 in which the dilute aqueous acid solution contains nitric acid as the sole acid ingredient. V

3. Process of claim. 1 in which the dilute aqueous acid solution consists of a dilute aquegusthsolution of a spent acid from a nitrating 4. Process for reducing the solution viscosity of regenerated cellulose which comprises treating pure comminuted regenerated cellulose sheeting with a dilute aqueous solution with an acid concentration of about 0.5% (expressed as nitric acid) and containing nitric acid as an essential ingredient, at a temperature of about C. for about 4 hours.

5. As a new product, regenerated cellulose having a cuprammonium viscosity of less than 2.0 compared to water as 1.0 prepared according to the method of claim 1.

6. As a new product, regenerated cellulose having a euprammonium viscosity of about 1.3 compared to water as 1.0 prepared according to the method of claim 1.

RALPH J. QUAID.

CERTIFICATE OF cpmcuon; Pate nt No. 2,268,092. necembe ab, 19m,

RALPK QUAID. It is hereby' certified thdf amp r appears in the pflntedipci-fication of the above numbered patient requiring :o 1 :'re t1on aa foliowaz PFE 5a a ec-r .ond colufin, line 2-6 for- "sci\p' fr ,'b.d 3crap page 14., second qdlumm' line 22, claim 1;, art' i'u uous' li sert --a1d--; and that the. said Let-- ters Patent should be r gdd. fliis correction-thera in phat'the same may conibi'm to the record of th s case in' the Patent: Office.

Signed and sealed this 31%! day r March, A. lat-19kg.

Henry van' gi'a'dgie, (Sea1) Acting Cohmiasibner of Patents". 

